Spider Web Architecture and Rainfall Damage: Observational and Manipulative Studies Along a Precipitation Gradient on the Tropical Andes

Rainfall is a key abiotic factor influencing species' overall fitness and their ecological interactions. Although rainfall's effects are expected to be generally positive, high-intensity rainfall can damage biological structures and thereby reduce fitness. As silk structures should be particularly vulnerable to heavy rainfall, web-building spiders can serve as ideal systems for examining the impacts of rainfall on organisms and their extended phenotypes. Here, we investigated how spider webs with different geometries-two-dimensional (2D) orbs and three-dimensional (3D) tangles and sheet-and-tangles-are affected by varying levels of rainfall intensity given both their physical structure and microhabitat use. We studied the impact of rainfall on spider webs along an elevational gradient on the eastern slopes of the Tropical Andes in Ecuador where rain intensity ranges from strong (> 4 mm/h) to mild (< 2 mm/h) from the lowland rainforests to higher-elevation cloud forests. We found that web damage significantly increased with rainfall intensity, but the amount of damage webs suffered differed for different web types. Orb webs, which are built in open microhabitats, had the highest probability of damage. Sheet-and-tangle webs, on the other hand, suffered the greatest material loss, likely due to their high silk content, despite being located in more protected microhabitats. A manipulative experiment showed that webs artificially protected from the rain suffered significantly less damage than those left unprotected, demonstrating the role of immediate cover in mitigating the impact of rainfall as a function of microhabitat use. Our findings demonstrate that spiders with different web architectures are differentially affected by heavy rainfall, thus highlighting the need to consider rainfall intensity as a factor determining the composition of web-building spider communities across precipitation gradients.